INTERNAL STRUCTURE. S'CictOn.—The theory and facts of ancestry related above are enforced by comparative anatomy, which finds in the structure of birds a close resemblance to the internal structure of reptiles. Birds, neverthe less, possess distinctive anatomical features fully entitling them to rank as a separate class. In general form the body is tapering forward from the shoulders to the head, with the feathers sloping backward. and again dimin ishing toward the tail; this is adaptive to prog ress through the thin media of air and water inhabited by birds; and the weight is thrown forward, securing proper balance for the nor mally horizontal position. In order to secure this there is a concentration of muscles and other organs in the region below the point of suspension of the wings. Not only are the fleshy portions of the legs mainly confined to the upper portions of these limbs, but the muscles which elevate the wings are actually placed on the under instead of the upper surface of the body.
Though the neck is sometimes much prolonged, the body is very compact and rigid. The back bone consists of vertebra. having (typically) sad dle-shaped, articular facets, and many processes and ligaments which lock them firmly together; moreover, the vertebra. of the back generally be come ankylosed or firmly united together by ce menting bone, the solidity thus acquired being for the support of the ribs, and these also are proportionately stronger than is usual in mam mals: each of them is provided in the middle with a flattened bony process, directed obliquely back ward to the next rib, so that they support one another. and they often Become ossified with the sternum, giving unusual rigidity and strength to the thoracic framework. The hinder part of the spine is consolidated with the pelvis. and the tail, primitively long, has heroine shortened into a few small movable vertebra terminating in a short and generally much elevated bone, consisting of ankylosed vertebra, called the pygo style, or plowshare bone.
In contrast to the general stiffness of the ver tebral column in the trunk, it is remarkable for great flexibility in the neck, enabling a bird to make ready use of its bill, or to bring its head into such positions as suit the adjustment of the centre of gravity in flying, standing, etc.; there are also certain peculiar ligamentous bands, by which birds can retain the neck in the nistomary S-eurve without. muscular exertion. The first (atlas) and second (axis) vertebrae are modified to form au articulation with the head. which is completed by a single globular condyle (as in reptiles), forming a sort of pivot, and enabling the head to be turned around with a freedom and to an extent impossible to the mam malia ; and the fact that it is formed almost wholly by the basioceipital bone constitutes a fun damental distinction between birds anal the rep tiles on one side and the mammals on the other.
The Skall.—The skull is formed of bones cor responding with those of man; but they can he distinguished only when the bird is very young, soon becoming consolidated together. The brain
case "is more arched and spacious, and is larger, in proportion to the face, than in any reptilia" except pterodactyls. The jaws are much elongat ed, so as to form the bill, the outward shape of which, however, formed by the horny sheath, may be very different from that of its bony supports, in adaptation to habits of use. The upper mandible is formed anteriorly and mainly of the prtemaxilbe, and posteriorly by the paired maxillaries and other bones, varying greatly in relative position and importance, and in many birds movable, so that the upper jaw is capable of a certain amount of up and down motion as on a hinge. These and other elements form a palate, the varying bony arrangement of which has been used with disputed success as a basis for general classification in ornithology. The lower jaw is formed of several elements now fused solidly together ; and it is connected with the skull by the quadrates and other lesser hones and by a series of elastic cartilages permitting extreme movability and large expansion of the Lcps.—Tbe limbs of birds conform to the ver tebrate type, with certain modifications. which are least in the hinder pair. They are attached to the 'pelvic arch,' which in birds consists of some solidified .aeral and coccygeal vertebrae and the paired pelvic bones ( ilium, ischium, and os pubis), which latter meet to form it cup in which rests the head of the femur. When one con siders how far behind the centre of gravity of a bird's body the hip-joint is placed, and how it must sustain the whole weight of the body under unfavorable circumstances, as well as provide for the leverage of the muscles of the thigh, the necessity for this consolidation, breadth, and mas siveness will he perceived. The thigh is short, and concealed within the body: the next division, often mistaken for the thigh, is the leg. strictly so called, or tibia and fibula, which ends at what is really the ankle-joint. although popularly re garden as the knee; and beneath this is the shank, commonly called the tarsus,whieh in some birds is very long, serving as a part not of the foot, but of the leg, and formed by a compound hone, composed of the united metatarsal bones, with the upper end of which four tarsal bones are fused. Thus the ankle-joint is not between the tarsus and metatarsus. hut between two series of tarsal bones. The tibia is therefore properly a tibiotarsus. and the so-called tarsus is really a tarsometatarsus. The feet are di vided into toes, which are four in number, three before and one behind, differing from each other in length and in the number of joints or phalanges of which they are composed. the hallux, which is directed backward, being in general compara tively short, and consisting of two joints only. No fifth toe is known, and the hallux (first toe) is absent in the Struthiones, which have lost also the second, while the fourth is diminished, throwing the work almost alone upon the third, or middle toe.